1. Field of the Invention
The field of the invention relates to display screens working notably in reflection mode. More specifically, the invention relates to composite materials based on polymers and liquid crystals containing dichroic dyes.
The working of this type of screen is based on the following electrooptical effect: a film made of composite material based on dyed liquid crystal and polymer is contained between two transparent conductive electrodes. The dyed liquid crystal is obtained by dilution of molecules of liquid crystal. At rest, without voltage, the film is light-scattering owing to the differences in refraction index between polymer and liquid crystal, and is furthermore absorbent owing to the molecules of dye whose orientation, which is dictated by that of the liquid crystal, varies randomly from one domain to the other. The application of a voltage to the two faces of this film causes the molecules of liquid crystal to get oriented with the molecules of chosen dichroic dye parallel to the applied electrical field. By working, on the one hand, with a liquid crystal whose ordinary index is equal to that of polymer (there is no scattering at the polymer/liquid crystal interface) and, on the other hand, with a dichroic dye that is no longer absorbent, a transparent film is obtained under an electrical field. A film such as this is therefore capable of having two states, a scattering and absorbent state and a transparent, non-absorbent state. The contrast obtained with composite materials such as these is far more satisfactory than that proposed with composite materials containing no dichroic dyes and having only a scattering state and a transparent state.
Furthermore, the use of composite materials based on dyed liquid crystal and polymer offers many advantages as compared with the other display techniques using notably liquid crystals. Indeed, these composite materials:
have the advantage by which polymers are easy to implement, which means that they can be easily deposited in thin layers, with controlled thicknesses, on large areas;
require no surface processing of the substrates forming the screen;
work without any polarizer (which implies a substantial gain in luminosity);
show an electrooptical effect that has an angle of view of the order of 150.degree..
2. Description of the Prior Art
Nevertheless, at present, the techniques used to implement these composite materials are not satisfactory, notably because of the presence of dichroic dyes.
* It is possible to use a method of emulsion of the dyed liquid crystal in an aqueous phase containing the hydrosoluble polymer or latex. However, the composite materials thus obtained require excessively high control voltages for an acceptable contrast. This is partly due to the differences in resistivity between the liquid crystal and the hydrosoluble polymer to which a substantial portion of the control voltage is directed, to the detriment of the liquid crystal that has to be oriented (Wiley R., WO 90/03593, 05.04.1990), (Drzaic P. S. Displays, 12.2.1991), (Drzaic P. S., Gonzales A. M., Jones P. Montoya W., SID 92 Digest, 571).
* At the same time, the methods of phase separation induced by thermal polymerization on a mixture of monomer, liquid crystal and dye show poor performance characteristics. These methods can only entail polycondensation and not radical polymerization owing to the deterioration of the dyes by the free radicals. However, in polymers obtained by polycondensation, a part of the dye is trapped in the amorphous chains of the polymer and not solely in the porous parts of the polymer. This is due to a degree of segregation between the polymer and liquid crystal that is generally insufficient because liquid crystal is a good solvent of the dye. The polymers typically used are epoxy-based polymers (West J. L., Ondris R., Erdmann M., SPIE LCD And Applications 76, 1990) and lead to very low contrasts.
* The methods of phase separation induced by photochemical polymerization (Ogawa T., Hotta S., EP 0434366 A2, Jun. 26 1991) are unsuitable owing to the low stability, under UV radiation, of certain molecules that form part of the composition of, the black dichroic dyes that deteriorate. Furthermore, the photopolymerization is obtained in a very incomplete way owing to the absorption of the photons by the dyes.